is heartbeat link over sub interface possible/feasable?

[dsykora]

We have an HA pair of Fortigate 200E.  Currently they are next to each other on a rack with direct cat6 RJ-45 patch cables between HA ports and port1 ports.  Soon we will move them to separate closets, and will need to connect them via fiber.  Since we have limited SFP ports and don't want to waste them on heartbeat ports, we want to reuse part of our WAN links as HA.  Currently we have an WAN switch in our dmarc that combines a variety of WAN circuits (each on separate vlan) and the tagged vlans are connected via SFP/fiber connections to the Fortigates.  We would like to add another vlan or similar sub interface and have a HA link run over that across the WAN switch.  We could run an HA link with media converters connected to RJ-45 Fortigate ports if needed, but don't really want to add extra media converters.

Thoughts?